A traveling wave tube is a vacuum device which serves as an amplifier of microwave frequency energy. It relies upon the interaction that occurs between an electron beam and a microwave signal. An electron gun at an input end of a slow wave structure (SWS) generates the electron beam. The electron beam travels along an axial path formed by the SWS. A microwave source inputs the microwave signal at the input end of the SWS. The microwave signal then propagates along the SWS toward an output end of the SWS.
The SWS causes the microwave signal to traverse an extended distance between two axially spaced points. This reduces the effective lateral propagation velocity of the microwave signal from that of light to that of the electron beam. Interaction between the electron beam and the microwave signal causes velocity modulation and bunching of the electrons in the beam. The interaction causes energy coupling to take place between the electron beam and the microwave signal that amplifies the signal. The amplified signal is then coupled out at the output end of the SWS.
Because of the close proximity between the electron beam and the SWS, part of the beam impinges the SWS and produces heat. The amount of heat generated also depends on the power of the electron beam and the microwave signal. If the traveling wave tube cannot remove the heat fast enough, the tube reaches a fairly high temperature. This fairly high temperature increases electrical resistance losses of the SWS and promotes the generation of gas. This, in turn, results in deterioration of the amplified microwave signal as well as of the electron beam transmission. Moreover, these undesirable phenomena reduce the service life of the traveling wave tube.
To mitigate the effects of heat, the traveling wave tube includes supporting rods to conduct the heat away from the SWS to a tube member which encloses the SWS. The supporting rods extend longitudinally adjacent the SWS and are located between the SWS and the tube member. In addition to conducting heat, the supporting rods support the SWS in the tube member.
Due to perturbance of the microwave signal on the electron beam and space charge effects arising from mutual repulsion between adjacent electrons, the beam tends to increase in diameter along the SWS. Thus, the traveling wave tube further includes a magnetic focusing device for constraining the electron beam along the axial path through the SWS to prevent excessive impingement of the electrons on the SWS. The magnetic focusing device generates a magnetic field which confines the electron beam.
A typical focusing device is a periodic permanent magnet (PPM) arrangement. The PPM arrangement includes a plurality of like short annular permanent magnets disposed in axial alignment along and about the SWS. A plurality of annular ferromagnetic pole pieces are interposed between and abut adjacent magnets. The magnets are magnetized axially and arranged with like poles of adjacent magnets confronting one another.
The amount of coupling between the electron beam and the microwave signal is approximately constant at low microwave signal input power levels. Thus, the gain between the microwave output and input signals is nearly constant. As the power of the microwave input signal increases, nonlinear effects become more significant. Eventually, the microwave output signal reaches a maximum power value and the traveling wave tube operates at saturation.
Approaching saturation, the gain between the microwave output and input signals starts to decline. If the power of the microwave input signal is increased further beyond saturation, the power of the microwave output signal and the gain decrease. A traveling wave tube operating below its saturated microwave output power is described as running backed off from saturation.
The power of the microwave output signal is also proportional to the electron beam power. Thus, saturation of the traveling wave tube occurs, regardless of the power of the microwave input signal, when the microwave output signal power is roughly 25% to 30% of the electron beam power.
The magnetic field strength of the PPM arrangement required for confining the electron beam is a function of the power of the microwave output signal. For instance, at saturation, the microwave signal significantly perturbs and effects the electron beam. Because of the significant perturbance and the space charge mutual repulsion effect, some of the electrons in the electron beam develop large radial velocity components. Accordingly, a strong magnetic field generated by a large number of magnets is needed to nullify the radial velocity components so that the electrons travel generally axially through the SWS without impinging the SWS.
On the other hand, running backed off saturation, the effect of the microwave signal on the electron beam is minimal. Thus, a weak magnetic field generated by some magnets is sufficient to nullify the radial velocity components caused by the space charge effects.
Typical traveling wave tubes are built to produce the desired saturated microwave output power and then are operated backed off from saturation to obtain the desired amplitude and phase linearity. This requires that the supporting rods be able to handle the full heat load generated by the electron beam and the microwave signal at saturation. The PPM arrangement also has to able to confine the electron beam at saturation. A primary disadvantage with typical traveling wave tubes is that if the tubes continuously run backed off from saturation, then the full capabilities of the supporting rods and the PPM arrangement are never utilized and are, therefore, not needed.